Method of recovery of natural gas liquids from natural gas at ngls recovery plants

ABSTRACT

A method to recover natural gas liquids from natural gas streams at NGL recovery plants. The present invention relates to methods using liquid natural gas (LNG) as an external source of stored cold energy to reduce the energy and improve the operation of NGL distillation columns. More particularly, the present invention provides methods to efficiently and economically achieve higher recoveries of natural gas liquids at NGL recovery plants.

FIELD

The present invention relates to methods for recovery of natural gas liquids (NGLs) from methane rich gases using liquid natural gas (LNG). More particularly, the present invention provides methods to efficiently and economically achieve higher recoveries of natural gas liquids at NGL recovery plants.

BACKGROUND

Natural gas from producing wells contain natural gas liquids (NGLs) that are commonly recovered. While some of the needed processing can be accomplished at or near the wellhead (field processing), the complete processing of natural gas takes place at gas processing plants, usually located in a natural gas producing region. In addition to processing done at the wellhead and at centralized processing plants, some final processing is also sometimes accomplished at ‘straddle plants’, These plants are located on major pipeline systems. Although the natural gas that arrives at these straddle plants is already of pipeline quality, there still exists quantities of NGLs, which are recovered at these straddle plants.

The straddle plants essentially recover all the propane and a large fraction of the ethane available from the gas before distribution to consumers. To remove NGLs, there are three common processes; Refrigeration, Lean Oil Absorption and Cryogenic.

The cryogenic processes are generally more economical to operate and more environmentally friendly, current technology generally favors the use of cryogenic processes over refrigeration and oil absorption processes. The first generation cryogenic plants were able to extract up to 70% of the ethane from the gas, modifications and improvements to these cryogenic processes overtime have allowed for much higher ethane recoveries >90%. This increase in recovery comes with consumption of relatively large quantities of energy due to their compression requirements. Prior art has taught that use of lean reflux streams reduce energy consumption and achieves high ethane recoveries, Moreover, methane gas has been proven to be a superior stripping gas to control carbon dioxide concentrations in NGL product. Many patents exist disclosing improved designs for generation of lean reflux to recover ethane and heavier components in NGL plants. they typically involve significant capital expenditures and increased operational costs. A need exists for an efficient ethane and NGL recovery process that is capable of achieving very high ethane recoveries at a lower energy consumption and a lower capital cost when compared to prior art.

SUMMARY

The present invention provides a method for recovery of natural gas liquids from natural gas streams in a NGL recovery plant. The method involves the use of LNG as a reflux stream, a feed mixer and a stripping gas in the operation of a LNG recovery plant. The use of LNG as stored cold energy to control a NGL distillation column temperature profile and operation, increases the efficiency and recovery of NGLs in natural gas streams. Moreover, LNG, primarily methane, is an ideal stripping gas to control carbon dioxide concentration in the NGL product stream.

As will hereinafter be further described, the interacting step can be either direct or indirect. Direct interaction is achieved by injecting LNG as a liquid reflux to the distillation column to control overhead temperature, by direct mix with expanded gas stream to control distillation column pressure and as a stripping gas for carbon dioxide control in NGL product stream. Indirect interaction is achieved by, first cooling the distillation column overhead stream in a heat exchanger and then used as a reflux in the distillation column. The condensate generated from overhead stream is used as a second reflux stream for a dual reflux operation, increasing NGLs recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:

FIG. 1 is a schematic diagram of a facility equipped with LNG storage and supply for direct cooling in accordance with the teachings of the present invention.

FIG. 2 is a schematic diagram of a facility equipped with LNG storage and supply for indirect cooling in a heat exchanger to generate a second reflux stream.

DETAILED DESCRIPTION

The method will now be described with reference to FIG. 1.

Referring to FIG. 1, a pressurized natural gas stream 1 is routed to heat exchanger 2 where the temperature of the feed gas stream is reduced by indirect heat exchange with counter-current cool streams 24, 19, 6 and 21. The cooled stream 1 enters feed separator 3 where it is separated into vapour and liquid phases. The liquid phase stream 4 is expanded through valve 5 and pre-heated in heat exchanger 2 prior to introduction into distillation column 20 through line 6. The gaseous stream 7 is routed to gas expander 8. The expanded and cooler vapor stream 9, is mixed with LNG for temperature control and routed through stream 17 into the upper section of distillation column 20. A LNG storage drum 10, supplies LNG through line 11 to LNG pump 12. The pressurized LNG stream 13 is routed through temperature control valve 14 providing the reflux stream to distillation column 20. A slipstream from the pressurized LNG stream 13 provides temperature control to stream 9 through temperature control valve 16, temperature controlled stream 17 enters the upper section of distillation column 20. The controlled temperature of stream 17 by addition of LNG enables operation of the distillation column at higher pressures to compensate for the loss of coolth energy generated by the expander at higher backpressures. A second slipstream from pressurized LNG stream 13 provides methane for carbon dioxide stripping through flow control valve 18, the LNG is pre-heated in heat exchanger 2 before introduction into the lower section of the distillation column 20 as a stripping gas. The distilled stream 21, primarily methane, is pre-heated in heat exchanger 2 and routed to compressor 22 for distribution and or recompression through line 23, The liquid fraction stream 24 is reboiled in heat exchanger 2 and routed back to the bottom section of distillation column 20, to control NGL product stream 25.

Referring to FIG. 2, the coolth energy of LNG is used to first condense the overhead stream of the distillation column generating a second reflux stream before its use as the primary reflux stream, allowing for an increase in efficiency in plant operations. A pressurized natural gas stream 1 is routed to heat exchanger 2 where the temperature of the feed gas stream is reduced by indirect heat exchange with counter-current cool streams 28, 18, 6 and 25. The cooled stream 1 enters feed separator) where it is separated into vapour and liquid phases. The liquid phase stream 4 is expanded through valve 5 and pre-heated in heat exchanger 2 prior to introduction into distillation column 19 through line 6. The gaseous stream 7 is routed to gas expander 8, the expanded and cooler vapor stream 9 is routed into the upper section of distillation column 20. A LNG storage drum 10, supplies LNG through line 11 to LNG pump 12. The pressurized LNG stream 13 enters heat exchanger 11 and is routed through temperature control valve 15 as reflux stream 16 to distillation column 19. A slipstream from pressurized LNG stream 13 provides methane for carbon dioxide stripping through flow control valve 17, the LNG is pre-heated in heat exchanger 2 before introduction into the lower section of the distillation column 19 as a stripping gas. The distilled stream 20, primarily methane, is cooled in heat exchanger 14 and discharged into overhead separator 21. The condensed stream 22 feed reflux pump 23, the pressurized reflux stream 24 enters distillation column 19 as a second reflux stream for a dual reflux distillation column operation. The vapour stream 25 is pre-heated in heat exchanger 2 and routed to compressor 26 for distribution and or recompression through line 27. The liquid fraction stream 28 is reboiled in heat exchanger 2 and routed back to the bottom section of distillation column 19, to control NGL product stream 29.

In the preferred method, LNG provides stored cold energy that improves the operation and efficiency of NGL distillation columns. The above described method uses this stored cold energy to condense natural gas liquids from natural gas streams by direct mixing. This direct mixing provides better heat transfer and reduces the energy requirements to condense NGLs. It also reduces the energy required for recompression of gas for distribution.

In this patent document, the word “comprising” is used in its non-limiting sense to mean that items the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.

The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. Those skilled in the art will appreciate that various adaptations and modifications of the described embodiments can be configured without departing, from the scope of the claims. The illustrated embodiments have been set forth only as examples and should not be taken as limiting the invention. It is to be understood that, within the scope of the following claims, the invention may be practiced other than as specifically illustrated and described. 

What is claimed is:
 1. A method for recovery of natural gas liquids from natural gas using the cold energy stored in LNG comprising the step of: The storage and supply of LNG as an external cooling source to control the operation and recovery of NGLs in a distillation column.
 2. The method as defined in claim 1, providing LNG as a reflux stream by a temperature control of the overhead gas stream by mixing of LNG with the rising gas stream in the distillation column.
 3. The method as defined in claim 1, providing LNG to directly mix with un-distilled expanded, feed gas to allow distillation column to operate at higher pressures without loss of recovery.
 4. The method as defined in claim 1, providing LNG as a stripping gas for carbon dioxide concentration in NGL product stream.
 5. method described in claim 1, providing LNG to cool an overhead stream to generate a second reflux stream for a dual reflux distillation column operation.
 6. A method for recovery of natural gas liquids from a natural gas comprising the steps of: positioning a storage vessel for liquid natural gas (LNG) at a NGL recovery plant facility that has at least one distillation column for recovering natural gas liquids (NGLs); adding LNG from the storage vessel by direct mixing to control the temperature profile in a NGL distillation column, the temperature in the overhead product of the distillation column being controlled by controlling addition of LNG as a reflux stream, the temperature in the expanded feed gas to the distillation column being controlled by controlling addition of LNG as a tempering gas, the stripping of carbon dioxide from the NGL product stream being controlled by controlling the addition of LNG as stripping gas.
 7. The NGL recovery plant as defined in claim 6, wherein LNG provides additional cooling energy to the inlet plant gas feed.
 8. NGL recovery plant as defined in claim 6, wherein the use of LNG as an external cold energy source is used to increase the overall energy efficiency and recovery of NGLs. 